This invention relates to labels. In one aspect, the invention relates to labels suitable for application to electronic components, e.g. an integrated circuit chip or a printed circuit board, while in another aspect, the invention relates to labels designed to dissipate static electricity that may be harmful to the electronic component. In yet another aspect, the invention relates to laminated labels comprising a backing film, a primer layer and a pressure-sensitive adhesive layer.
Static dissipation is important for electronic components which are vulnerable to damage from very low voltage (e.g. 50 V) discharges. For example, computer board assemblies contain many static sensitive integrated circuit chips which bear barcode labels that are used for tracking and identification of the boards. These labels are potential sources of static electricity.
Static electricity is generated during application and removal of a label by a phenomenon known as triboelectric charging. Whenever two insulative surfaces rub against one another or are separated from each other, a charge imbalance is generated on each of the surfaces. Since the surfaces are insulative, these charges are not dissipated and thus build to an eventual discharge (which usually appears as a spark). These discharges can destroy the gate oxide layers inside of an integrated chip, thus rendering it useless. Even low voltage discharges which do not generate a visible spark can destroy a modern integrated circuit.
The typical label currently in use for electronic components comprises a backing film one side of which is coated with a pressure-sensitive adhesive and the other side of which is coated with a printable topcoat. The pressure-sensitive adhesive affixes the label to the electronic part while the printable topcoat carries tracking and identifying information about the part. The label is typically provided with a silicone or other suitable liner to protect the pressure-sensitive adhesive until the label is ready for application to the part.
All of the materials from which the label is built are generally insulative or nonconductive in nature. Static electricity is generated at the time the label is peeled from the liner before application to the electronic part, and these charges can exceed hundreds of thousands of volts. During the peeling operation, a danger exists that these charges will discharge and damage the part in the vicinity at which the label is applied. The repositioning or removal of the label is a second triboelectric charging event that also carries the danger of discharge.
To avoid or reduce the risk of these triboelectric charging events, preferably the label is constructed from conductive materials. However since only the adhesive is involved in the peeling process, only the adhesive stores the charge. If the adhesive is conductive, the charge can be dissipated harmlessly.
The standard method of imparting conductivity to an insulative adhesive is to incorporate conductive particles into the adhesive to a loading sufficient to give particle-to-particle contact. However, this is typically accomplished at the cost of adhesiveness loss, i.e. at such conductive particle loadings, the stickiness of the adhesive is compromised.